This invention relates to a method and apparatus for drilling boreholes in the course of geological exploration for, and exploitation of, hydrocarbons and their by-products. More particularly, the invention relates to directional and horizontal drilling. Conventional geological exploration techniques have involved drilling vertical holes ("straight-holes").
Recent advances in art of drilling include the use of multiple high-angle development wells and directional and horizontal drilling techniques. High-angle well techniques involve drilling a well into a discovered oilsand reservoir with the drillstring inclined at a substantial angle from the vertical. Directional drilling involves drilling a first borehole leg, a transition zone and a second borehole leg inclined at a substantial angle from the first borehole leg, so as to interpenetrate and exploit multiple oil-bearing sands from a single bore. For example, in horizontal drilling, the first leg may be vertical, and the second leg may be substantially horizontal, with a transition zone therebetween. The transition zone at which the two legs of the borehole meet may range from a gradual curve to an abrupt bend. The severity of the transition zone is measured in either bend radius or angle of inclination per horizontal distance. Thus, a transition zone curving at 2-6.degree./100 ft (3000-1000 ft radius) is regarded as a "long radius" borehole, whereas a transition zone of 1.5-3.degree./ft (40-20 ft radius) is regarded as a "short radius" borehole. While these advances in drilling techniques have increased well output over conventional straight-hole drilling methods, they have engendered a host of practical difficulties and imposed increased mechanical duty on drillstring components.
The mechanical duty imposed on drillstring components--the assembly of drill pipes, joints, drill collars and bit--by the advanced drilling techniques includes increased material fatigue due to high-maqnitude stress reversals. A conventional drill collar, heavyweight drill pipe, or drill pipe consists of a high-grade steel tubular cylinder of standard length (10-30 ft depending upon the application), having a circular cross section and a concentric passageway at the center for pumping a slurry/lubricant, referred to in the trade as "drilling mud", to the drill bit.
When used in high angle drilling operations, the inclination of the drillstring creates gravitationally induced bending moments along the drillstring component spans. These bending force amplitudes are further increased by the inability of the drill collars to provide uniform tension on the drillstring when subjected to a gravitational force component which is not in-line with the drill collar longitudinal axis. Consequently, the flexure, or bending moments manifest themselves as stress gradients across the diameter of the drillstring, inducing a compressive stress component in the upper half of the drillstring component and a tensile stress component in the lower half of the drillstring component. Each rotation of the drillstring during drilling subjects the drillstring component material to a flexure reversal of the stress field, leading to substantially increased mechanical fatigue of the drillstring components relative to conventional straight-hole drilling techniques.
Of equal importance to the increased mechanical duty is the reduction in volumetric efficiency encountered in high-angle and horizontal wells, i.e., the efficiency with which cuttings are removed from the borehole. Drilling mud--a rheolitic slurry of fluid and buoyant suspension agent, e.g., bentonite--is pumped through a passageway in the drillstring to the bit, where it is injected at high velocity and pressure against the formation through jets located in the bit. The heavy consistency of the drilling mud captures the cuttings generated by the bit, while its buoyant character causes the cuttings to rise out of the path of the bit. Because the drill bit diameter exceeds that of the other drillstring components, the cutting-laden drilling mud rises to the surface in the annulus surrounding the drillstring. It has been observed that in high-angle wells, there is a tendency for the cuttings to settle toward the lower side of the bore, due to the influence of gravity, thereby reducing the efficiency of the drilling mud in cleaning the hole. The problem with solids settling out of the drilling mud is exacerbated as the well angle increases, becoming most critical for horizontal boreholes.
Reduction in volumetric efficiency attributable to reduced effectiveness of the drilling mud hole-cleaning ability in high-angle and horizontal wells impacts a number of parameters. Because the cuttings are not removed from the path of the drill bit quickly, drilling efficiency (the rate of penetration or ROP) is reduced, leading to increased drilling time and energy requirements to achieve a specified borehole depth. Additionally, energy is lost by grinding the cuttings remaining in the path of the drill bit. The effect increases the difficulty in removing the cuttings and decreases the useful life of the bit--a substantial consideration in costly diamond drilling bit applications. Moreover, frequent removals of the drillstring to replace worn bits is a time consuming and expensive process, increasing the risk of a blowout endangering personnel.
Yet another important problem encountered in drilling oil and gas wells is the phenomena of "differential sticking." Differential sticking occurs when the fluid in the drilling mud located in the drillstring-borehole annulus is absorbed unevenly around the periphery of the drillstring through the porous media of the borehole wall. This fluid loss induces a pressure differential across the drillstring diameter which causes the drillstring to be deflected against the borehole wall on the side experiencing the fluid loss, and can lead to halting engagement of the drillstring against the borehole wall. Once so engaged, the unbalanced fluid pressure acts to keep the drillstring in engagement with the borehole wall. The torque required to free the drillstring may exceed the capacity of the rotary table or the top drive used to drive the drillstring, or may exceed the yield strength of a drillstring component, leading to "twistoff" (torsion induced fracture). Differential sticking may result in the loss of the drill bit and a portion of the drillstring, thereby necessitating time consuming and extremely expensive procedures to recover the detached drillstring portion. In some cases, where the detached portion cannot be retrieved, the drill operator may have to abandon the borehole and begin anew.
A final phenomenon observed with conventional drillstrings is that of "key seating" at "doglegs" (borehole direction changes) and "kick-off points", i.e., locations at which the angle of attack of the drill bit and drillstring is altered as the inclination from the vertical is increased. The phenomena of key-seating arises when there is sufficient bend in the borehole path to cause a portion of the drillstring to come into contact with one side of the borehole wall. This contact, if not substantial enough to cause differential sticking, can result in the drillstring forming a groove approximately the diameter of the drillstring in the borehole wall. If viewed in cross-section perpendicular to the borehole longitudinal axis, the borehole and groove would resemble a keyhole, with a large lower portion and a narrower upper portion. When key-seating occurs, it may no longer be possible to withdraw the drillstring from the borehole, since the larger diameter elements of the drillstring assembly (drill collars, stabilizers, etc.) will be unable to pass through the narrow groove. The phenomena of key-seating is due in large part to the rigidity of conventional drillstring components, which are unable to provide enough flexure to accommodate borehole directional changes and changes in the angle of attack. As with differential sticking, key-seating can lead to twistoff, necessitating time consuming retrieval procedures or abandonment of the borehole.
The aforementioned problems have provided a fertile ground for invention, and a number of prior art drillstring component designs are directed toward resolving one or more of these problems. One solution adopted by a number of prior art drillstring components, including the present invention, is the use of a helical flat or groove around the periphery of the drillstring component. Prior art drillstring components using such a solution may be generally grouped into two categories, each characterized by a disadvantage that the present invention is designed to overcome.
A first category of prior art helical groove drillstring component employs screw-like threads or broad V-shaped notches. Fitch U.S. Pat. No. 3,085,639 discloses a drill collar having screw-like threads on its periphery for drilling straight boreholes, wherein the flights of the screw coact with the borehole as a screw conveyor in removing cuttings from the vicinity of the drill bit. Arnold U.S. Pat. No. 3,194,331 and Massey U.S. Pat. No. 3,360,960 disclose, respectively, drillstring components having a single and multistep V-shaped helical groove on the circumference designed to reduce differential sticking, increase drilling mud flow through the borehole-drillstring annulus, and to act as a broach to reduce key-seating.
In operation, the configuration of the helical groove in all three of these patents is such that the sharp edges of the grooves may strip the drilling mud lining the borehole wall (referred to as wallcake), leading to instability of the borehole wall and concomitant loss of fluid from the borehole. The drillstring component of the present invention is designed to leave intact the desired wallcake thickness, generally 3/32", while still providing superior performance by increasing drilling mud flow up the annulus, plus reducing differential sticking and key-seating.
A second category of helical groove drillstring component employs a spiral groove wherein the groove constitutes essentially a chord intersecting two points on the circumference of the drillstring component. Fox U.S. Pat. No. 2,999,552, Chance et al. U.S. Pat. No. 4,460,202, and Hill et al. U.S. Pat. No. 4,811,800 all disclose spiral groove drillstring components wherein the groove forms a chord on the component, when viewed in transverse section. The purpose of the groove is to reduce differential sticking, improve flow of drilling mud up the borehole-drillstring annulus and to increase the load on the drill bit in directional drilling applications. Hill et al. U.S. Pat. No. 4,811,800 discloses trading-off drillstring component service life in favor of increased drillstring flexibility by employing a relatively deep spiral chord-style groove. The drillstring component of the present invention is designed to provide the benefits attributed to these prior art chord-style spiral groove drillstring components, plus superior service life and flexibility in short radius directional drilling applications.
In view of the foregoing, it is an object of this invention to provide a drillstring component for drilling high angle and short radius directional and horizontal boreholes which experiences reduced mechanical fatigue duty relative to previously known drillstring components, and which is readily integrable with existing drilling systems, including downhole drilling mud-driven turbine style motors ("mudmotors").
It is a further object of this invention to provide a drillstring component for drilling high angle, directional and horizontal boreholes which improves volumetric and drilling efficiencies, reduces time and energy costs of drilling, and increases drill bit life relative to that achieved with previously known drillstring components.
It is another object of this invention to provide a drillstring component for drilling high angle, directional and horizontal boreholes which substantially reduces the incidence of differential sticking, thereby reducing the major costs associated with retrieval of detached drillstring portions or abandonment of a partially drilled well.
It is yet another object of this invention to provide a drillstring component for drilling high angle and horizontal boreholes which has adequate flexibility to reduce the costs and additional effort required by incidents of key-seating and possible twistoff of the lower portion of the drillstring.
It is still another object of this invention to utilize the rotary motion of the drillstring to induce a turbine-style pumping ("turbo-pumping") action of the cutting-laden drilling mud away from the drill bit and subterranean formation interface toward the drilling mud treatment equipment at the borehole entrance.
This invention includes method steps carried out in sequence for obtaining the desired borehole-cleaning capability when drilling high angle, directional and horizontal boreholes.